Fanon is basically anything non-canon that is generally accepted as "fact" or at least a plausible possibility by fandom (or a subset of fandom, such as a particular group of shippers). The distinguishing characteristic of pure fanon is that it has no direct canon support -- it can perhaps be inferred from canon, but it is not. Supported. By canon.

There is some fanon that I readily accept and even love -- shit, my OTP is pure fanon, as are most pairings out there. Harry and Draco are never going to get together in canon (if they do, I'll be the first checking into a mental hospital, or perhaps escorting JKR there). However, there is subtext which slashers the world over have used to put them (and pretty much all other characters) together in believable and convincing ways. That's wonderful.

Some well-written "missing moments" fics create such an interpretation of characters and events that convinces most readers that's that's how it must have happened. For instance, fernwithy's Of A Sort creates such a brilliant portrayal of the Sorting of Albus Dumbledore, Minerva McGonagall, Tom Riddle, and other adult characters (whose Sortings we've never seen in canon and likely never will) that I personally accept as fact. They're just so well thought out and steeped in canon that there is no reason for me to try and reinvent the wheel. Of course, this also requires that I accept Fern's interpretation of the characters (which I do), but some people won't agree and will create their own versions of fanon for these characters' Sortings, and that's wonderful.

However. There is the fanon that has zero basis in canon but is accepted by a variety of glomping fangirls and even serious fans alike -- and most of the time it's because it increases the sex appeal of characters. Since I prefer to write/rant/talk about what I know, example: Draco the Ice Prince of Slytherin.

WHAT THE FUCK? Every time I see a fic which characterises Draco as this "cool-as-a-cucumber aristocrat" I want to scream. Really loudly. WHAT CANON ARE YOU PEOPLE READING TO ACCEPT SUCH AN IDIOTIC INTERPRETATION OF DRACO MALFOY? It's really fangirly wish-fulfillment at its best. Draco Malfoy is a disagreeable little shit. He gets in Harry's face when he has the opportunity to taunt him about something. He clowns around for the older Slytherins when he's a kid, pulling grotesque faces and repeating lame jokes until nobody wants to hear them. THAT IS NOT ICE PRINCE BEHAVIOUR, MKAY? Yes, Draco is perhaps better at self-control than Harry but not nearly so much as to turn him into an unflappable icicle.

Second example: Draco in leather trousers. OMGWTFCASSEROLE? Draco Malfoy is a wizard. They do not wear Muggle clothing. DRACO MALFOY HATES MUGGLES AND MUGGLEBORNS, OR AT LEAST THERE IS NO CANON INDICATION TO THE CONTRARY! Wizards in the books frequently have trouble with Muggle clothing, and a poncy pure-blood like Malfoy is about as likely to be caught wearing Muggle clothes for fun as he is likely to get caught collecting plugs, for crying out loud. Draco Malfoy is not Arthur Weasley. Duh. Stop playing dress-up with the characters, there are Barbie dolls for that.

Third example: Quidditch-toned muscles. OH MY GOD. TO THE PEOPLE WHO USE THIS IN FIC: WHAT THE FUCK ARE YOU SMOKING AND CAN I HAVE SOME? Harry Potter is not muscular due to his madd Quidditch skillz, dudes. He SITS on a freakin' BROOM and occasionally stretches out his arm to catch a tiny flying ball. Maximum what he's going to get from that is blisters on his arse, not ripped abs. HARRY POTTER IS SKINNY AND HE HAS KNOBBLY KNEES. STFU AND DEAL. I wonder if the person who invented Quidditch-toned muscles has ever actually tried working out. I suppose that's a rather wanky thing to say, but honestly. Quidditch-toned muscles. Wtf, dude.

There are numerous other fanon "facts" that have arisen from popular fics that deserve to die a slow and painful death (the fanon facts, not the fics). Just off-hand, slut!Seamus, suave!Snape, sex god!Lucius, etc ad nauseam. What I've observed is that the majority of such fanon serves to increase/augment the sex appeal of characters, and you all know my views on the overabundance of shipfic in the fandom. Oh, you don't? Well then. >:)

THERE IS TOO MUCH FUCKING LOVE AND ROMANCE IN THIS FUCKING FANDOM! DON'T THESE PEOPLE DO ANYTHING BUT SHAG, EVER? WHAT THE FUCK HAPPENED TO THE ADVENTURE? THE DRAMA? THE MYSTERY? THE INTRIGUE? The Harry Potter fandom is blessed with an unbelievably rich source material that's just teeming with possiblities for amazing fiction that explores the world and characters further.

I am in no way opposed to smut, don't get me wrong. I don't think it's wrong or immoral or has no place in a children's fandom omg. It's not a fucking children's fandom, JKR herself has said that she's not writing these books for kids. Smut is fantastic. But it'd be nice to see more about LIFE in fanfic, not just pornography and FEELINGS. We all have feelings, but not all of those feelings center around romance. WHERE IS THE FAMILY LOVE THAT IS NOT INCESTUOUS? WHERE IS THE FRIENDSHIP?

NOW GO AND WRITE SOME GENFIC! GIVE ME NEW MINISTRY DEPARTMENTS AND EXCITING SPY STORIES OR GIVE ME DEATH!